This invention relates to a metal containing waste water treatment method and treatment equipment, which collects gallium and arsenic from a waste water that contains gallium as a valuable metal and arsenic as a poisonous metal and recycles the substances. This invention further relates to a metal containing waste water treatment method and treatment equipment, which can constitute a completely closed treatment system of a (gallium arsenide) waste water or the like from compound semiconductor plant by carrying out pretreatment appropriate for the remaining treated water to recycle the resulting liquid as a raw water for an ultrapure water generating system. This invention further relates to a metal containing waste water treatment method for providing a completely closed treatment system of a (gallium arsenide, gallium phosphide and so on) waste water from compound semiconductor plant by separately collecting (1) gallium and (2) a mixture of arsenic and phosphorus from the waste water from compound semiconductor plant that contains gallium as a valuable metal, arsenic as a poisonous metal and so on, recycling the substances in another place and carrying out pretreatment optimum for the remaining water to recycle the resulting liquid as a raw water for an ultrapure water generating system.
Conventionally, it has been the most general way to treat the gallium arsenide containing waste water by the so-called neutralizing coagulation and sedimentation method for treating the waste water with ferric chloride as a coagulant, a neutralizer and a high molecular coagulant added. According to this neutralizing coagulation and sedimentation method, a sedimentation has been treated and disposed of as an industrial waste under the legal regulations.
On the other hand, there is a method for collecting a concentrate by evaporating and concentrating the gallium arsenide containing waste water, restoring the evaporated water into water by cooling and carrying out appropriate pretreatment to recycle the resulting liquid as a raw water for an ultrapure water generating system.
There is another prior art disclosed in Japanese Patent Laid-Open Publication No. 2000-117270. This prior art adjusts the pH of a metal containing waste water with an alkali agent of sodium hydroxide or the like, forms a metalhydroxide, thereafter makes the waste water pass through a membrane separation unit that has a pore diameter of 1 mm to 10 mm to thereby efficiently separate water from metalhydroxide and collect and recycle the valuable metal. Further, as a pretreatment process, the pH of the metal containing waste water is adjusted to 3 to 4, hydroxides of iron and chromium are separated and collected to selectively separate and collect the metals.
As shown in concrete in FIG. 58, this prior art treatment equipment disclosed in Japanese Patent Laid-Open Publication No. 2000-117270 is constructed of a pH adjustment tank 941, an MF membrane separation unit 942 filled with a ceramic membrane or the like, a pump 943, a pH readjustment tank 944, an RO membrane separation unit 945 and a redissolution tank 946.
Then, waste water, which contains heavy metals, is supplied so that a residence time in the pH adjustment tank 941 becomes 30 minutes. Subsequently, the pump 943 connected to the MF membrane separation unit 942 is operated.
The pump 943 of the treated water is interlocked with a liquid level switch placed in the pH adjustment tank 941 and controlled by the water level of the pH adjustment tank 941. The metalhydroxide generated in the pH adjustment tank 941 is concentrated by a film placed in the MF membrane separation unit 942.
On the other hand, the concentrated metalhydroxide is dissolved in the redissolution tank 946 and becomes a high-concentration nickel and zinc solution to be recycled in a plating bath and the like of the plant.
Another prior art disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 2000-117270 will be described with reference to FIG. 59. This prior art treatment unit is constructed of a ferrooxidans bacterium reaction tank 1048, an MF membrane separation unit 1049, a pH adjustment tank 1041, an MF membrane separation unit 1042, a pH readjustment tank 1044, an RO membrane separation unit 1045 and so on.
Then, the pH of the waste water in the ferrooxidans bacterium reaction tank 1048 is adjusted to pH 3 with sulfuric acid and sodium hydroxide, while nitrogen and phosphorus are added as a nutrient. A ferrooxidans bacterium collecting MF membrane separation unit 1049 is placed inside this ferrooxidans bacterium reaction tank 1048. An MF membrane that is made of silica alumina-based ceramics and has a pore diameter of 10 μm is employed for this MF membrane separation unit 1049. This MF membrane has its membrane surface continuously cleaned from inside the membrane by air.
Part of the concentrate brine of iron hydroxide, chromium hydroxide and the ferrooxidans bacterium, generated by this MF membrane, is sent back to the ferrooxidans bacterium reaction tank 1048, and another part is extracted, dried, granulated and thereafter recycled. Inside this ferrooxidans bacterium reaction tank 1048, iron hydroxide, chromium hydroxide and the ferrooxidans bacteria are accumulated and controlled within an MLSS (Mixed Liquor Suspended Solids) concentration from 100 to 200 mg/l (milligrams/liter). Further, the pH of the ferrooxidans bacterium treated water is adjusted to 9 by a sodium hydroxide solution in the pH adjustment tank 1041. Then, after stirring, a hydroxide of nickel and zinc is generated.
Subsequently, the water is made to pass from the pH adjustment tank 1041 to the MF membrane separation unit 1042 placed outside the pH adjustment tank 1041 by the pump 1043.
Yet another prior art (third prior art) is disclosed in Japanese Patent Laid-Open Publication No. HEI 9-285786. According to this third prior art, as shown in FIG. 60, mixed water 1175 obtained by preliminarily adding a chemical for precipitating arsenic or an adsorbent for adsorbing arsenic to raw water that contains arsenic is flowed into a membrane filter tank 1174.
Otherwise, arsenic in the raw water is precipitated by making the raw water flow into the membrane filter tank 1174 and making mixed water by adding the aforementioned chemical or adsorbent. Otherwise, adsorption to the adsorbent is effected, and solid-liquid separation of mixed water 1176 inside the membrane filter tank 1174 is effected by a membrane filter 1173 (submerged membrane) placed inside the tank 1174. In the above case, the precipitation of arsenic is promoted by setting the amount of a membrane filtered water 1177 with respect to the amount of influent water into the membrane filter tank 1174 to 99% or more and maintaining the added chemical at high concentration inside the membrane filter tank 1174. In FIG. 60 are shown a raw water introducing pipe 1171, a coagulant loading pipe 1172, a treatment tank 1175 and a water tank 1188.
This third prior art oxidizes trivalent arsenic into pentavalent arsenic by either one of a method for adding an oxidizer or a method for carrying out ozone treatment, thereafter introduces the mixed water to which the chemical or the adsorbent has been added into the membrane filter tank 1174 in which the submerged membrane is placed and separates arsenic by promoting the precipitation or adsorption of arsenic.
Still another prior art (fourth prior art) is disclosed in Japanese Patent No. HEI 3-61514. This fourth prior art is a waste water treatment method for removing arsenic from a waste water that contains gallium and arsenic and collecting gallium.
In concrete, soluble ferric salt is added to the waste water that contains gallium and arsenic, and gallium and arsenic are coprecipitated with the precipitation of ferric hydroxide by adjusting the pH with an alkali agent.
Then, the precipitate is suspended in water, and gallium is eluted from the precipitate by adjusting the pH to the alkali side with sodium hydroxide, or an alkali agent added. After separation from the precipitate, water is evaporated and hardened by drying to collect gallium.
The aforementioned prior art of Japanese Patent Laid-Open Publication No. 2000-117270 describes the “metal containing waste water treatment and valuable metal collecting method characterized by forming a metalhydroxide by adjusting the pH of the metal containing waste water, thereafter making the resulting liquid pass through a membrane separation unit that has a pore diameter of 1 mm to 10 mm, thereby separating water from metalhydroxide”.
In contrast to this, according to the gallium arsenide process in a semiconductor plant, the liquid is circulated in the unit using a filter that has a pore diameter of not greater than 1 μm (pore diameter of 0.4 μm, for example) in a back polishing unit, or production equipment in the plant. Therefore, particles of not smaller than 0.4 μm are discharged in the waste water. In order to catch the particles of not smaller than 0.4 μm, it is required to make the water pass through a membrane separation unit that has a pore diameter of 0.4 μm.
On the other hand, it is required to carefully examine the pore diameter of the MF membrane to be adopted considering the fact that the general MF membrane has a pore diameter of about 1 μm to 10 μm and the fact that the pump power is increased when the pore diameter is reduced, also in consideration of energy saving.
Moreover, there are availed on the market UF's (Ultra Filter: ultrafiltration membrane) that have a pore diameter of about 0.002 μm to 0.4 μm. Therefore, it is proper to select the model conforming to the purpose and construct an energy-saving system.
The prior art disclosed in Japanese Patent Laid-Open Publication No. 2000-117270 cannot collect metal at high concentration (10% or more) and discharges metal ions collected by a reverse osmosis membrane in the waste water without recycling the metal ions. In the MF membrane separation unit of Japanese Patent Laid-Open Publication No. 2000-117270, the electrical energy consumed by the pump is large.
The aforementioned prior art (Japanese Patent Laid-Open Publication No. 2000-117270) describes the “metal containing waste water treatment and valuable metal collecting method characterized by forming a metalhydroxide by adjusting the pH of the metal containing waste water, concurrently loading a high molecular coagulant or a liquid chelating agent to form a flock of metalhydroxide and thereafter making the resulting liquid pass through a membrane separation unit that has a pore diameter of 50 μm to 200 μm, thereby separating water from the flock of metalhydroxide”.
Although a concession is made to the fact that this description is intended for hydroxide, it is desired that the impurities of coagulant and so on should not be included, in order to collect gallium, or the valuable metal. In concrete, although only the gallium hydroxide is desired to be included, the high molecular coagulant or the liquid chelating agent is concurrently loaded to form a flock of metalhydroxide according to this prior art.
Furthermore, the aforementioned prior art (Japanese Patent Laid-Open Publication No. 2000-117270) describes the “metal containing waste water treatment and valuable metal collecting method characterized by forming a hydroxide of chromium and iron from the metal containing waste water that contains ions of trivalent chromium and bivalent iron through the oxidation of the bivalent iron to trivalent iron with pH adjusted to 3 to 4 in the first stage, thereafter making the resulting liquid pass through a membrane separation unit that has a pore diameter of 1 μm to 10 μm to thereby separate water from the hydroxides of iron and chromium and next separating and collecting the remaining metals from the treated water in the second stage according to the methods claimed in claim 1 or 2”.
If the gallium arsenide waste water is treated by applying this prior art, there is the problem that the arsenic is oxidized from trivalent arsenic to pentavalent arsenic and easily precipitated and becomes a mixture of hydroxides of arsenic and gallium as a precipitate, causing a trouble in refining and collecting gallium.
There is a cost merit when the gallium hydroxide and the pentavalent arsenic are independently separately collected to the utmost.
Moreover, the aforementioned prior art (Japanese Patent Laid-Open Publication No. 2000-117270) describes the “metal containing waste water treatment and valuable metal collecting method as claimed in claim 3, characterized in that a ferrooxidans bacterium is used in oxidizing the bivalent iron to the trivalent iron”. In contrast to this, there is the problem that the ferrooxidans bacterium is not useful when treating the gallium arsenide waste water.
Moreover, the aforementioned prior art (Japanese Patent Laid-Open Publication No. 2000-117270) describes the “metal containing waste water treatment and valuable metal collecting method characterized in that, when metals are separated and collected from the metal containing waste water that contains ions of nickel, zinc, trivalent chromium and bivalent iron according to the method as claimed in claim 3 or 4, hydroxides of nickel and zinc are formed by adjusting the pH to 8 to 10 in the second stage, separating water from the hydroxides of nickel and zinc.
However, in contrast to the requirement of the formation of a precipitate by adding a coagulant after the oxidation of arsenic in the second stage, i.e., after arsenic is made to be a stable insoluble salt in the case of the gallium arsenide waste water, the aforementioned prior art has the problem that no oxidation process exists.
Moreover, the aforementioned prior art (Japanese Patent Laid-Open Publication No. 2000-117270) describes the “metal containing waste water treatment and valuable metal collecting method as claimed in any one of claims 1 through 5, characterized in that a membrane made of ceramics is employed as a membrane separation unit”. However, the membrane separation unit made of ceramics is generally expensive.
Moreover, the aforementioned prior art (Japanese Patent Laid-Open Publication No. 2000-117270) describes the “metal containing waste water treatment and valuable metal collecting method as claimed in any one of claims 1 through 6, characterized in that the metal of the concentrate of the metalhydroxide that has been separated and collected is re-dissolved by adjusting the pH to 0.5 to 3 by sulfuric acid, and the concentrate brine of the metal is collected and recycled.
However, there is a further problem that a concentration and precipitation process is needed after the membrane separation of the prior art since there is required a gallium slurry of which the gallium concentration is as high as possible when gallium is refined from the gallium arsenide waste water.
Moreover, the aforementioned prior art (Japanese Patent Laid-Open Publication No. 2000-117270) describes the “metal containing waste water treatment and valuable metal collecting method as claimed in any one of claims 1 through 7, characterized in that the treated water from which the metalhydroxide has been separated and collected is recycled by making the treated water pass through a reverse osmosis membrane.”
However, in contrast to the necessity of a pretreatment system before the gallium arsenide waste water is made to pass through the reverse osmosis membrane, the prior art has no pretreatment process.
Moreover, the aforementioned prior art (Japanese Patent Laid-Open Publication No. 2000-117270) has the problem that the power of the pump (pump 1043 and pump 1050) related to the MF membrane separation unit is great now that energy saving is valued.
The third prior art (Japanese Patent Laid-Open Publication No. HEI 9-285786) discloses in the claim 1 the “water treatment equipment operating method by means of an immersion type membrane filter unit characterized in that the precipitation or adsorption of arsenic is promoted by flowing a mixed water obtained by preliminarily adding a chemical for precipitating arsenic or an adsorbent for adsorbing arsenic to a raw water that contains arsenic into a membrane filter tank or by flowing the raw water into the membrane filter tank then forming a mixed water with the added chemical or adsorbent, thereby precipitating the arsenic in the raw water or adsorbing the arsenic to the adsorbent, and effecting solid-liquid separation of the mixed water inside the membrane filter tank by the immersion type membrane filter unit placed in the tank and setting the amount of membrane filtered water with respect to the amount of influent water into the membrane filter tank to 99% or more when the membrane filtered water that has passed through the membrane surface of the immersion type membrane filter unit is unloaded from the tank, thereby maintaining the chemical or the adsorbent at high concentration in the membrane filter tank”.
In this claim 1, although arsenic can be treated, there is no process for separating two metals apart from each other as in the case of the gallium arsenide waste water. Moreover, the claim 2 claims the “water treatment equipment operating method by means of the immersion type membrane filter unit claimed in claim 1, characterized in that the arsenic in the raw water is oxidized to the pentavalent arsenic ion by either adding an oxidizer to the raw water or treating the raw water with ozone”. This claim 2, which employs the chemical for oxidizing the arsenic in the raw water to the pentavalent arsenic ion, requires a chemical cost.